PerspectivePlanetary Science

Generating an Atmosphere

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Science  24 Dec 2010:
Vol. 330, Issue 6012, pp. 1755-1756
DOI: 10.1126/science.1200473

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The presence of water ice on most of the large satellites of the outer planets was established many years ago through near-infrared (1- to 2.5-µm wavelength) observations with ground-based telescopes. Frozen carbon dioxide, sulfur dioxide, methane, nitrogen, and other molecular ices are also found in various combinations on inner planets such as Mars to bodies far beyond Pluto. Recent discoveries of ice varieties on some asteroids and sequestered in protected regions on Mercury and the Moon point to the near-universal distribution of frozen volatiles throughout the solar system (13). On page 1813 of this issue, Teolis et al. (4) report the detection of a tenuous (approximately one in five trillionth of Earth's atmospheric pressure at sea level) oxygen (O2) and carbon dioxide (CO2) atmosphere surrounding Saturn's icy moon Rhea (diameter of 1529 km) measured as the Cassini spacecraft passed by only 97 km above the surface in March 2010 (see the figure).